1. Field of the Invention
The present invention relates to composite cast iron alloys and particularly to composite cast iron alloys that are made from melted cast iron and melted aluminum.
2. Prior Art
Conventionally, the most common material for low pressure casting equipment for aluminum or aluminum alloy is Ferric Cast Iron (FC) 20-25. As a consequence, in the melt of the aluminum alloy, etc., impurities such as carbon and iron components which are derived from the FC cast iron are mixed in, resulting in lower quality of the casting products of aluminum alloy.
The degradation in product quality mentioned above is not merely due to the mixing of the FC 20-25 cast iron into the melt of the aluminum alloy, etc., but other reasons such as corrosion by electric current also account for some of it. Such a current is generated by an electrochemical reaction taking place due to the creation of a local battery. Accordingly, as a preventive measure, coating or lining over the surface of the crucibles or stalks with various types of highy anticorrosive materials have been attempted. However, a material with a satisfactory effect has not been found as yet. For example, for durability alone, a coating of silicon carbide of silicon nitride or the formation of a ceramic layer by flame spray among other methods, will accomplish this purpose. However, these coated layers are unable to maintain their normal hot strength because of the mixing of fluorides and chlorides used as a slag remover in the aluminum melt. Hence, at the present stage, these measures have not resulted in the effect expected.
In search of a means to prevent the melting out and the corrosion of crucibles and stalks, inventors took note of metallic titanium and found out that a satisfactory result can be obtained for stalks by insertion casting with metallic titanium pipes. Such a proposal is the basis for Japanese Patent Application No. 1981-112656. Metallic titanium is high in wear resistance, corrosion resistance as well as heat resistance. In addition, even if it is fused into the aluminum melt, it brings about a refining effect on the crystal grain of the aluminum alloy. Therefore, an improvement in mechanical as well as physical properties of the aluminum metal alloy can be expected. In fact, such effect was actually obtained, thus ensuring the desirability of the metallic titanium. Also, the service life of the stalk was increased to nearly thirty days which is twice as long as a stalk using ordinary FC cast iron as the material since its service life is not more than fourteen days.
Nevertheless, observation of the conditions of the stalk made by insert casting with metallic titanium after its use revealed the following fact. That is, instead of the wear of the metallic titanium plate itself being the greater problem, corrosion due to the exposed surface of the FC cast iron due to breakage in the lining layer caused the greater damage. This fact indicates that the corrosion caused by the separation of the titanium lining layer is a serious factor. The reason for the breakage is assumed to be the hysteresis expansion caused by the large growth phenomenon of ordinary cast iron during the heating and cooling processes. Consequently, the inventors came to conclude that unless this hysteresis expansion is prevented, sufficient improvement in the product by means of insertion casting with titanium metal cannot be obtained.
In an attempt to solve the corrosion problem, the inventors continued their pursuit for a solution be selecting various cast iron alloys and by casting stalks from conventionally known cast iron alloys such as high aluminum cast iron, Al-Si system Alsiron cast iron and Cralfer cast iron that is obtained by adding Cr to the former. The actual operation test conducted on stalks obtained with the above alloys showed a desirable effect for each of them in terms of performance. Also, the service life of the stalks could be extended to nearly twenty days. However, the inventors were not successful in finding an aluminum cast iron alloy having a service life longer than twenty days.